You can find this exercise on page 304 of the book Synergistic Golf.
Exercise Type: HEAVY BALL
Exercise Category: HIPS, LEGS - FEET, STRENGTH
Kneel on all fours with heavy ball behind feet. Lift body into pushup position, then slowly lift one foot then the other onto ball. Toes may be pointed or flexed. Body should maintain flat back. Evenly disperse weight between hands and feet. Hold with controlled breathing for 10-30 seconds.